This invention relates to zinc oxide particles having a cover layer formed of zinc silicate on the surface and hence having not only greatly suppressed surface activity and high shielding ability of ultraviolet (UV) rays inclusive of those of a long wavelength region around 400 nm but also high transparency to visible light. The invention further relates to a process for production of such zinc oxide particles.
In recent years, the increase of UV radiation which reaches to the surface of the earth or the biosphere as a result of the depletion of the ozone layer has come to public notice as a contributing factor to the damage to the human health.
Under these circumstances, the interest in the protection of the human body from UV rays is accordingly increasing. As already known, there are two regions of UV rays reaching the surface of the earth or the biosphere, B-region UV rays (UV-B) at wavelengths of 290-320 nm and A-region UV rays (UV-A) at wavelengths of 320-400 nm. More recently, there is growing a great interest in the protection of the human body from UV-A rays at longer wavelengths as well as UV-B rays.
Organic UV-absorbers have been conventionally used in resin films, coating compositions (i.e., paints) or sunscreen cosmetics. However, the organic UV-absorber has a problem of bleeding. That is, it migrates to the surface layer from the inside of resin molds such as resin films or coatings as time passes. The organic UV-absorber has a further problem that its UV shielding effect diminishes as time passes owing to the photodecomposition.
Furthermore, the conventional organic UV-absorbers are stimulative to the human skin when they are used in cosmetics, and in addition, they have a limited spectrum of UV rays which they can absorb. Thus, there is now a great demand for a material which is able to shield a broader UV spectrum.
On the other hand, inorganic UV-absorbers have been also heretofore widely used, especially ultrafine particles of rutile titanium dioxide, in the field of coating compositions or cosmetics. However, rutile titanium dioxide has a large refractive index (2.7) so that it has a fair hiding power in spite of the fact that they are adjusted to be ultrafine in their particle size.
Accordingly, when rutile titanium dioxide is incorporated in resin molds or coating compositions, it is poor in transparency to visible light. Further, when a sunscreen agent which contains rutile titanium dioxide therein is applied to the human skin, the titanium dioxide scatters blue light strongly so that the skin often looks pale and unhealthy. As a further problem, the rutile titanium dioxide is not enough in shielding effect of UV-A rays.
To cope with these problems, a composite oxide composed of Ti, Si and Fe was proposed that is excellent in shielding ability of UV-A rays (JP-A-9-30933). Iron-containing rutile titanium dioxide ultrafine particles were also proposed as a useful material for sun blocking cosmetics which look less pale when being applied to the human skin (JP-A-5-330825). However, these modified titanium dioxides are still insufficient in shielding ability of UV-A rays. Further, these modified titanium dioxides have still a large refractive index so that they have a tendency to hide the human skin when they are applied to the human skin. Accordingly, if they are incorporated in sunscreen cosmetics in such an amount that they can shield effectively UV-A rays, they have no transparent appearance when being applied to the human skin.
Ultrafine particles of zinc oxide have an absorption edge at a wavelength of 380 nm, and they are able to shield most part of UV-A rays as well as UV-B rays, and moreover, they have a small refractive index (2.0) and highly transparent. Because the ultrafine particles of zinc oxide are as such, they attract a great deal of attention as a UV-absorber in recent years. Thus, they have been heretofore used mainly in the field of cosmetics, but they are now spreading their application in various fields on account of their excellent properties in addition to cosmetics.
However, on the other hand, various problems have been indicated when the known ultrafine zinc oxide particles are used as UV-absorbers. For example, when they are used in thermoplastic resins such as polyethylene terephthalate or polycarbonate, they promote the decomposition of these resin, and deteriorate their moldability remarkably. When the known ultrafine zinc oxide particles are used in coating compositions, the zinc oxide reacts with a binder resin in the compositions to increase the viscosity of the composition and to gel the composition as time passes.
Furthermore, when the resin molds or coatings which contain the known ultrafine zinc oxide particles therein are exposed outdoors, there is a problem that the zinc oxide decomposes photocatalytically the resins around the zinc oxide particles. With regard to cosmetics containing the known ultrafine zinc oxide particles therein, there is a fear that reactive oxygen species are photocatalytically generated at the surface of the particles.
Zinc oxide has a further property that it is inherently soluble slightly in water and, as is known, the physiological effect of the thus dissolved zinc ion has been utilized as astringent in the field of cosmetics from long ago. The chemical reactivity of zinc oxide to saponify fatty acids to form metal soaps is often utilized to absorb sebum secreted by the human skin to improve makeup wear, or as deodorant to absorb the body odor components.
These physiological activity and the chemical reactivity of zinc oxide become stronger as it is finer in particle size and accordingly, it is demanded to restrain the surface activity of zinc oxide particles, in particular, ultrafine zinc oxide particles, from the viewpoint of its safety for the human skin.
To meet the demand as mentioned above, the present inventors already proposed to treat the surface of zinc oxide particles with high density silica in order to restrain the surface activity which zinc oxide inherently possesses (JP-A-11-336316). However, such a method as surface treating or coating forms a cover layer formed of hydrated oxides such as hydrated silica or alumina on the surface of zinc oxide particles. Accordingly, when zinc oxide particles are treated by such a method and incorporated in high temperature molding resins or high temperature baking coating compositions such as powdery coating compositions or precoat metals, there arises a problem that dull finish of resin molds or coatings results due to the coating layer formed of hydrated oxides on the surface of zinc oxide particles as mentioned above.
The thus hydrated oxide-coated zinc oxide particles have also a fatal defect that they promote the hydrolysis of resins such as polyethylene terephthalate or polycarbonate resin.
In addition, food packaging film must shield UV rays at wavelengths up to of about 400 nm depending upon the kind of foods therein, and an topcoat layer of two-coat automobile coating system composed of top coats and primer coats must prevent the transmission of UV rays at wavelengths of up to about 400 nm or more. However, ultrafine zinc oxide particles alone fail to shield the rays at wavelengths of more than 380 nm.
The invention has been completed in order to solve the problems involved in the zinc oxide particles known heretofore, in particular, the known ultrafine zinc oxide particles.
Accordingly, it is an object of the invention to provide zinc oxide particles, preferably ultrafine zinc oxide particles, which have suppressed surface activity and give safe and effective UV ray shielding ability to a variety of products such as resin molds, coating compositions or cosmetics free from the problems as mentioned hereinabove.
It is a further object of the invention to provide a process for the production of such ultrafine zinc oxide particles having suppressed surface activity.
It is also an object of the invention to provide uses of such ultrafine zinc oxide particles as above mentioned, particularly resin molds, coating compositions or cosmetics.
It is still an object of the invention to provide ultrafine zinc oxide particles which are able to shield UV rays at longer wavelengths of about 400 nm.
The invention provides a particulate zinc oxide or a zinc oxide particle having suppressed surface activity characterized in that it has on the surface a cover layer formed of zinc silicate in an amount of 0.5-50% by weight, preferably 1-15% by weight, in terms of zinc silicate (zinc orthosilicate, Zn2SiO4) relative to the zinc oxide.
Herein the invention, when the terms xe2x80x9cin terms of zinc silicatexe2x80x9d are used, the xe2x80x9czinc silicatexe2x80x9d means zinc orthosilicate, Zn2SiO4.
According to the invention, water-insoluble zinc compounds which are converted to zinc oxide by heating in the air can be used as raw materials (i.e., starting materials) for the zinc oxide particles of the invention, as mentioned in detail hereinafter, as well as particles of zinc oxide itself. Water-soluble zinc salts which form the above-mentioned water-insoluble zinc compounds when they are neutralized with a neutralizing agent can be also used as starting materials particularly when particles of zinc oxide which have not only solid solution iron or cobalt atoms therein but also a cover layer formed of zinc silicate on the surface are to be obtained, as mentioned in detail hereinafter. In these cases, when the terms xe2x80x9crelative to the zinc oxidexe2x80x9d are used, they mean xe2x80x9crelative to the zinc oxidexe2x80x9d which is formedxe2x80x94in a theoretical amount, strictly speakingxe2x80x94from the above-mentioned water-insoluble zinc compounds or water-soluble zinc salts used as raw materials for zinc oxide in a manner as mentioned above.
The zinc oxide particles which have suppressed surface activity are obtainable by adding aqueous solutions of a water-soluble silicate and a water-soluble zinc salt to an aqueous suspension of particles of water-insoluble zinc compound in such a chemically stoichiometric ratio so that they form zinc silicate (Zn2SiO4) as well as in an amount of 0.5-50% by weight in terms of zinc silicate (Zn2SiO4) relative to the zinc oxide, washing with water and drying the resulting product, and calcining the product at a temperature of 300-1200xc2x0 C. thereby forming zinc oxide particles and a cover layer formed of zinc silicate on the surface of the particles.
There are used as the above-mentioned water-insoluble zinc compound, for example, zinc oxide, zinc hydroxide, zinc carbonate, basic zinc carbonate, zinc sulfide or zinc oxalate.
When zinc oxide particles which contain iron or cobalt atoms therein in the form of solid solution are to be obtained, prior to the addition of aqueous solutions of a water-soluble silicate and a water-soluble zinc salt to an aqueous suspension of particles of water-insoluble zinc compound in such a chemically stoichiometric ratio so that they form zinc silicate (Zn2SiO4) in the process mentioned above, particles of zinc compound which have hydrated iron oxide or cobalt oxide precipitates on the surface in a predetermined amount relative to the zinc oxide (to be formed) are first prepared. Then, aqueous solutions of a water-soluble silicate and a water-soluble zinc salt are added to an aqueous suspension of particles of water-insoluble zinc compound in such a chemically stoichiometric ratio so that they form zinc silicate (Zn2SiO4), and the resulting product is washed with water and dried, and then the product is calcined at a temperature of 300-1200xc2x0 C.
In this way, the zinc compound particles are converted to zinc oxide particles, that is, zinc oxide particles are formed from the zinc compound particles and iron or cobalt atoms are diffused into the thus formed zinc oxide particles to form a solid solution while forming a cover layer of zinc silicate on the surface of the zinc oxide particles.
It is usually necessary to calcine the zinc oxide particles at a temperature at least of several hundred degrees, preferably at a temperature of as high as of 600-1000xc2x0 C. in order that iron or cobalt atoms form a solid solution in the crystals of zinc oxide particles. However, when zinc oxide particles are heated at such a high temperature, the particles grow excessively to fail to provide zinc oxide particles of high transparence. Nevertheless, according to the invention, zinc oxide particles do not sinter to form large particles if they are heated at such a high temperature because of the cover layer formed of zinc silicate on the surface, and hence ultrafine zinc oxide particles are obtained containing solid solution iron or cobalt therein.
The invention further provides a particulate zinc oxide or zinc oxide particle having suppressed surface activity characterized in that it has a first cover layer on the surface comprising zinc silicate in an amount of 0.5-50% by weight in terms of zinc silicate (Zn2SiO4) relative to the zinc oxide while the particle may contain iron or cobalt atoms in the form of solid solution, and a second coating layer comprising oxides of at least one element selected from the group consisting of Al, Si, Zr, Sn, Sb and rare earth elements in an amount of 0.5-30% by weight in terms of oxides relative to the zinc oxide on the first cover layer.
The above zinc oxide particle is obtained by preparing zinc oxide particle having a cover layer formed of zinc silicate on the surface while it may contain solid solution iron or cobalt, adding an aqueous solution of water-soluble compound of at least one elements selected from the group consisting of aluminum, silicon, tin, zirconium, antimony and rare earth elements to an aqueous suspension of the zinc oxide particle, adding an acid or an alkali as a neutralization agent to neutralize the water-soluble compound and precipitate on the surface of the zinc oxide particle thereby forming the second coating layer on the zinc oxide particle.
The zinc oxide particle of the invention as mentioned above may be further treated with organosilicon compounds, higher fatty acids, higher fatty acid esters, metal soaps, polyhydric alcohols or alkanolamines, if necessary.
The zinc oxide particle of the invention have an average particle size preferably of not more than 0.15 xcexcm.